1. Field of Invention
This invention relates to the detection and measurement of selected fluids present in expired air associated with the respiration of a living subject. More specifically, the invention pertains to determination of expiration rates or concentration of a selected fluid constituent in a fraction of expired air taken from a living subject during respiration.
2. Description of Prior Art
The detection and measurement of various fluids constituents present in the expired air of a living subject can provide valuable information with regard to numerous physiological processes in addition to respiratory functions. Values for oxygen consumption rate (VO.sub.2) and CO.sub.2 production rate (VCO.sub.2) serve as timely indicators of relative changes in cardiovascular function and tissue perfusion in critically-ill patients. Such values also provide the basis for calculation of metabolic rate, a parameter which is particularly significant in burn patients whose metabolic rate may increase by fifty to three hundred percent. Such patients must be closely monitored in view of the common increase of catbolism of protein and associated loss of body weight resulting from the break down of tissue required to supply energy for such dramatic metabolic requirements. Furthermore, VO.sub.2 and VCO.sub.2 values are useful in the calculation of energy expenditure for a patient in connection with surgery, infection or injury. An awareness of metabolic rate can provide an accurate basis for dietary planning to insure that calorie intake is properly coordinated to avoid lipogenesis or other adverse physiological consequences of excess calorie consumption.
Although instrumentation is available to obtain VO.sub.2 and VCO.sub.2 measurements, such instrumentation is typically complex and expensive. Current apparatus for monitoring VCO.sub.2, for example, generally utilizes a programmable calculator or computer, a flow metering device (ultrasonic, turbine, differential pressure, oscillating bellows) and an infrared gas analyzer, a mass spectrometer or a gas chromatograph. In such a system, the expired gases are either collected in a spirometer or are passed through a flow meter and integrated electronically to obtain an exhaled minute volume.
The CO.sub.2 concentration of the exhaled volume is measured with one of the aforementioned CO.sub.2 sensors and VCO.sub.2 is accordingly calculated. Such systems are not only extremely expensive, but are cumbersome to move and require expert attendance for accurate operation. Part of the cause for such complexity and expense arises from the approach by such prior art systems to make direct quantitative measurement of the CO.sub.2 fluid constituent. Such direct measurement have not only resulted in complex and expensive CO.sub.2 measurement devices, but have tended to limit progress in the art of detection and quantitative analysis of other fluid constituents occurring during respiration.